Just brought home a cutlass ciera.
Has a tranny leak from the cooling line, am working on that.
140k miles, had the goofy cheap rubber pan gasket you get with the discount store filter kit. So it was changed at least once.
Brown fluid, grey with silt. Was probably run so low on fluid it started slipping then topped off.
I changed the filter, fluid, gasket, and put an adjustable modulator on it. Cranked the screw on the modulator to make more pressure on the clutches. Put in a little bars leaks tranny fluid conditioner, STP like syrup to thicken it up a little.
This trans was slipping like crazy once it left 1st gear. Had to really nurse it.
Now it holds solid, or, dare I say it, better than new. (Programmed to be slushy to please old people.)
Though old fluid has some grey stuff in it it also loses viscosity and bleeds around the o-rings. I would strongly assume many trannies have problems start in the valve bodies, not being able to apply the appropriate pressure to the clutches, which then wears them out. It takes time though so if you notice tranny issues for heavens sake handle them early! I saved a trans that many would say was pretty far gone and a shop probably would have said "rebuild"!
If you really want to band aid an old trans use type F where dexron is called for, it slips less. But my generic "dex/M" is doing quite nicely.